Study On Control Of Grain Boundary Microstructure And Its Effect On Magnetic Properties In 2:17 Type Sm Co Magnets

2:17 type SmCo magnet is one kind of important magnetic material due to its superior magnetic properties,temperature stability and corrosion resistance.This type of magnet can not only be used in the elevated temperature environment,but also has promising potential to substitute for heavy rare-earth-doped Nd-Fe-B magnets.It has been known that the superior magnetic properties of 2:17 type SmCo magnets primarily derive from the unique nanostructured cellular microstructure and the researches so far have mainly focused on the microstructure in the grain interiors and its effect on the magnetic properties.However,the grain boundary areas also play an important role in determining the magnetic properties.The studies on grain boundary are relatively scarce and there still exists controversy about the effect of grain boundary microstructure on magnetic properties.Based on the investigation of effect of grain boundary microstructure on magnetization behavior and magnetization reversal process,this dissertation proposes a method to manipulate the grain boundary microstructure via doping CuO powders,and the microstructure,magnetization reversal process and the relationship between grain boundary microstructure and magnetic properties induced by doping CuO are systematically investigated,in which the pinning behavior of magnetic domain walls and its relation to cellular microstructure are intensively discussed.The dissertation firstly investigates the microstructure of grain boundary and the related magnetic behaviors.It is shown that Cu is lean at the grain boundary leading to the absence of Sm（Co,Cu）₅ cell boundary phases in the grain boundary areas and therefore deteriorating the continuity and integrity of the cellular microstructure.Through observation of magnetic domain reversal with in-situ MOKE（magneto-optical Kerr effect）microscope,it is found that the Cu-lean grain boundaries are vulnerable in magnetization reversal and the reversed magnetic domains can propagate from the periphery of grain boundaries into the grain interiors,and thereby resulting in low coercivity and squareness in the hysteresis loop.High resolution LTEM（Lorentz transmission electron microscopy）observation shows differences in magnetic domain wall motion between the grain boundary area and the grain interior,unveiling the reason of the multi-stage phenomenon in the initial magnetization curve of the studied SmCo magnets.The researches of grain boundary microstructure and the related magnetic behaviors enrich the understanding of magnetic properties of SmCo magnets and provide the basic idea to control of the microstructure of grain boundary.On the basis of the investigation of Cu-lean grain boundary,doping CuO powders is adapted in Sm₂₅Co₄₈Fe₂₀Cu₅Zr₂ magnets to control the microstructure of the grain boundary.It is found that the grain boundary area becomes Cu-rich and more complete cellular microstructure forms in the grain boundary areas after CuO doping.As a result,the reversed magnetic domains do not easily emerge and expand in grain boundary areas,and the enlargements of reversed magnetic domains in grain boundary and grain interior are independent.With the help of modification of grain boundary,the coercivity therefore has been enhanced from 945 kA/m to 1904 kA/m with doping 0.4 wt.%CuO.From the studies on the manipulation of grain boundary microstructure and the magnetization reversal process,it is expected to extend the method of controlling grain boundary microstructure in other kind of SmCo magnets,and enrich the understanding of magnetization reversal process of SmCo magnets.In order to deepen the understanding of the relationship between grain boundary structure,magnetic domain structure and magnetic property,micromagnetic simulations are carried out.It is found that the lack of the complete cell boundary phase deteriorates the pinning strength considerably and therefore the coercivity.The dependence of the profile of magneto-crystalline anisotropy constant around the cell boundary phase on the magnetic domain wall pinning behavior is investigated.It is shown that the interface between cell interior phase and cell boundary phase can act as pinning site of magnetic domain wall;the slope of magneto-crystalline anisotropy constant in the cell boundary phase has obvious influence on the pinning strength,i.e.,larger slope resulting in larger pinning strength.With a dip profile of magneto-crystalline anisotropy constant,the depth and width of the dip in the centre of the cell boundary phase would lead to non-monotonic change of pinning strengths,and the decrease of magneto-crystalline anisotropy constant at the cell boundary phase would decrease the pinning strength.